The present invention relates to a clamping fixture for releasably locating a workpiece pallet at a chuck.
Such clamping fixtures are put to use preferably for clamping workpiece pallets provided with a tool or workpiece positionally defined, the chuck usually being fixedly located on a work table of a machine tool or a press whilst the workpiece pallet is repeatedly releasably located precisely positioned at the chuck.
In semi- or fully automated machining it is important that the pallet mounting the workpiece being machined is correctly clamped in place on the chuck or totally released therefrom. If the pallet, for example, is not correctly located on the chuck, there is a risk of it shifting the workpiece out of place during machining, for instance during milling, with ruinous consequences. When the pallet needs to be lifted off the chuck after machining it must be assured that the locking mechanism is totally released so that the pallet can be safely unchucked. This is particularly important where heavy pallets are concerned, i.e.—so heavy that they cannot be lifted manually but, instead, require the assistance of a robotic or hoist fixture.
It will readily be appreciated that lifting off a pallet with the clamping members still not fully released can be likewise ruinous and cost-prohibitive. One problem in detecting whether the locking mechanism is fully locked or released is that a central component or central sensor is incapable of recognizing whether individual clamping members are truly locked or released. On top of this, recourse is made usually to monitoring auxiliary variables such as, for example, the pneumatic pressure needed for locking or release in an attempt to detect whether the locking mechanism is working correctly. This proves to be highly problematic, however, because it fails to always ensure a reliable indication as to whether the clamping members are in the extended locked position or in the retracted released position. Should the clamping members tend to become jammed in thus failing to become fully released or locked, then it is very difficult, even impossible, to recognize this by sensing auxiliary variables.
Known from German patent DE 196 36 375 is a quick-release fixture for a pallet. The chuck serving to receive a clamping spigot, termed “pull-in nipple”, arranged on the pallet is provided with a feedthru bore. The clamping spigot features knife-like contact surfaces having the task of parting chips and the like resulting from machining. The locking mechanism of the chuck is provided with a plurality of locking pistons serving to fixedly locate the clamping spigot. At its inner, radial end each locking piston features a stop which in the locked condition is seated on a heel of the clamping spigot, non-positively retaining the clamping spigot in the feedthru of the chuck. The stop doubles as a knife for likewise parting ingressed chips. The locking pistons comprise axial feedthru bores, but the document makes no mention whatsoever for what purpose these bores are provided. The chuck is provided with a plurality of ports for jetting it clean.
German patent DD 26733 discloses a chuck with a means for checking workpiece location. The chuck in the form of a vise features two jaws, each drilled with a feedthru hole, both of which are connected by flexible tubings to a stationary base part of the chuck in which a central feedthru is drilled which is connected by a horizontal bore to the two flexible tubings. In the horizontal bore two throttle valves are incorporated. For monitoring the pressure in the tubings and bores pressure sensing means are provided. Clamping a workpiece in place results in an increase in pressure in the branch of the tubing assigned to the corresponding jaw by the workpiece in snug contact within the jaws as can be detected by means of the assigned pressure sensing means.